It is known to actuate vehicle occupant protection devices in response to a vehicle crash event. Typically, one or more crash event sensors are attached to the vehicle for detecting the occurrence of the vehicle crash event. For example, the crash event sensor may be an inertia sensing device, a crush sensing device, and/or an acceleration sensing device. When the crash event sensor or sensors detect the occurrence of a crash event, one or more occupant protection devices are actuated to help protect one or more vehicle occupants.
Another type of occupant protection system transmits signals that are reflected off an adjacent object. The distance between the vehicle and adjacent object is determined by a time interval for the signal to return. For example, the system may use electromagnetic waves or ultrasonic waves to measure the distance between the vehicle and an adjacent object. The system attempts to prevent the occurrence of a vehicle crash event by activating a warning signal to alert a vehicle occupant when the measured distance indicates an increased risk of a vehicle crash event.